The invention is directed to an antenna with reconfigurable polarization, and more particularly, to an antenna having a perturbed square ring slot configuration for operating in multiple polarizations.
Circular polarized (CP) antennas are popular choices in mobile wireless communications applications owing to their ability to allow flexible orientation between the transmitter and receiver antennas and to reduce multipath effects that can lead to signal fading, e.g. as described in S. H. Hsu and K. Chang, “A Novel Reconfigurable Microstrip Antenna with Switchable Circular Polarization”, IEEE Antennas and Wireless Propagation Let., Vol. 6, 2007, pp. 160-162; Y. J. Sung, T. U. Jang, and Y. S. Kim, “A Reconfigurable Microstrip Antenna for Switchable Polarization”, IEEE Microwave & Wireless Components Let., Vol. 14, November 2004, pp. 534-536 (hereinafter “Sung”); and S. T. Fang, “A Novel Polarization Diversity Antenna for WLAN Applications”, Antennas and Prop. Society International Symposium, Vol. 1, 16-21 Jul. 2000, pp. 282-285. The ability to efficiently operate with both senses of CP (LHCP and RHCP) allows the system to reuse frequencies and double the system capacity, e.g. as described in F. Yang and Y. Rahmat-Samii, “A Reconfigurable Patch Antenna Using Switchable Slots for Circular Polarization Diversity”, IEEE Microwave and Wireless Components Letters, Vol. 12, No. 3, March 2002, pp. 96-98 (hereinafter “Yang”). Moreover, if the antenna can be switched between two senses of CP as well as linear polarization, it will allow the user to roam to virtually any existing network, as described in Sung.
A printed circuit realization is ideal for wireless applications due to low profile, simple fabrication, low cost, and compatibility with integrated circuits. A common technique for achieving circular polarization is to feed the antenna in two locations with a 90 degree phase shift between the antenna ports. This technique has the drawbacks of requiring two feed lines as well as a hybrid network of some kind to provide the necessary phase shift. Single feed circular polarization has been realized in microstrip antennas through the introduction of a perturbation in opposing corners of the antenna, e.g. as described in M. Niroojazi and M. N. Azarmanesh, “Practical Design of Single Feed Truncated Corner Microstrip Antenna”, Proceedings of the Second Annual Conference on Communication Networks and Services Research, Volume 00, pp. 25-29; P. C. Sharma and Kuldip C. Gupta, “Analysis and Optimized Design of Single Feed Circularly Polarized Microstrip Antennas”, IEEE Transactions on Antennas and Propagation, Vol. AP-31, No. 6, November 1983, pp. 949-955; and P. C. Sharma and K. C. Gupta, “Optimized Design of Single Feed Circularly Polarized Microstrip Patch Antennas”, Antennas and Propagation Society International Symposium, Volume 20, May 1982, pp. 156-159. These perturbations introduce a second near-degenerate mode. If the antenna is fed correctly, these modes can be generated with the same amplitude and a 90 degree phase difference resulting in CP. In these designs, the polarization is either RHCP or LHCP depending on the relationship between the feeding microstrip line and the truncated corners.
While traditional microstrip antennas provide a limited CP bandwidth, printed slot antennas can be more attractive elements in some cases because they provide an improved operating bandwidth without increasing the overall size of the element, e.g. as described in J. S. Row, “The Design of A Squarer-Ring Slot Antenna for Circular Polarization”, IEEE Transactions on Antennas and Propagation, Vol. 53, No. 6, June 2005, pp. 1967-1972. These elements can then be perturbed to provide a wideband CP element by applying the principle of complementary structures to the previously mentioned perturbed class of microstrip structures, as described in R. M. Sorbello and A. I. Zaghloul, “Wideband, High-Efficiency, Circularly Polarized Slot Elements”, Antennas and Propagation Society International Symposium, Vol. 3, 26-30 Jun. 1989, pp. 1473-1476.
Antennas with reconfigurable polarization have been a popular topic in the literature due to their applications in wireless communications devices. Fries, Grani, and Vahldieck presented an annular slot antenna with switchable polarization in “A Reconfigurable Slot Antenna With Switchable Polarization”, IEEE Microwave and Wireless Components Letters”, Vol. 13, No. 11, November 2003, pp. 490-492 (hereinafter “Fries”). An illustration taken from Fries is shown in FIG. 1. The authors present two configurations for this antenna. One antenna can switch between LHCP and RHCP. The other antenna configuration allows switching between either LHCP or RHCP and LP, but not all three states. Thus, each antenna can operate in a maximum of two polarization states.
Sung presents an antenna capable of switching between LHCP, RHCP, and LP by biasing PIN diode switches to select the desired truncations on a microstrip patch antenna. This design, shown in FIG. 2, operates with a bandwidth of less than 2%. Yang describes a microstrip antenna with polarization diversity, but their design is limited in switching between LHCP and RHCP, with no operation possible in LP.
There have been patents issued for antennas with switchable polarization. C. C. Liu, “Low Profile TEM Mode Slot Array Antenna”, U.S. Pat. No. 5,596,336, issued 21 Jan. 1997, describes a slot array capable of switching polarizations. However, this design requires a polarizing screen to achieve CP, introducing an extra layer of complexity. Y. T. Lo, “Multifunctional Microstrip Antennas”, U.S. Pat. No. 4,728,960, issued 1 Mar. 1988, describes multifunction microstrip antenna utilizing truncated corners, but these antennas do not have the ability to switch polarizations. FIG. 3 illustrates the antenna described in F. G. Farrar and D. H. Schaubert, “Selectable-mode microstrip antenna and selectable-mode microstrip antenna arrays”, U.S. Pat. No. 4,379,296, issued 5 Apr. 1983, where the PIN diodes function as shorting posts at the desired locations and do not effectively change the shape of the radiator.
It would therefore be desirable to provide a reconfigurable antenna without such disadvantages.